Chemical Aging of Semivolatile Secondary Organic Aerosol Sesquiterpene Products

The oxidation of biogenic volatile organic compounds (bVOCs) is a significant source of secondary organic aerosols (SOA). This study investigates later-generation SOA formation from the reactions of three first-generation sesquiterpene ozonolysis products (β-nocaryophyllinic acid, 2-(2-carboxyethyl)-3,3-dimethylcyclobutane-carboxylic acid, and 2-(2-methyl-6-oxoheptan-3-yl)-3,6-dioxoheptanal) with the OH radical. These compounds were synthesized specifically for this study. Our results demonstrate that sesquiterpene-derived SOA can become progressively more oxidized as it reacts with OH, with an average oxygen-to-carbon (O:C) ratio of approximately 0.6, in contrast with previous studies, suggesting considerably lower ratios. As the first-generation products continue to react, the corresponding SOA aerosol mass spectrometer (AMS) mass spectrum changes by 20-25° compared to that of the fresh SOA. This suggests that quantifying the sesquiterpene SOA under ambient conditions based on its ozonolysis products may be problematic. To account for these observations, we propose and test a physicochemical model describing the processes that convert first-generation sesquiterpene SOA into highly oxidized compounds. Our analysis suggests that approximately two-thirds of carbon in the ozonolysis SOA follows functionalization pathways. The parametrizations developed in this work can be utilized in future modeling efforts to reassess the contribution of sesquiterpenes to SOA formation.